Recently, attempts have been made to make sub-combustion chambers in diesel engines of ceramic material A, as shown in FIGS. 1 and 2, to improve the thermal resistance as well as the thermal efficiency of the sub-combustion chambers. Si.sub.3 N.sub.4 is a main component of a ceramic material that has been used and has substantially the same thermal conductivity as a metal cap for the chamber. Therefore, a method has been proposed in which a metal casing B is fitted onto an outer circumference of a ceramic body through an adiabatic air gap D to raise the combustion temperature by utilizing the thermal resistance of the ceramic material.
In FIG. 1, a metal casing B is shrunk-fit onto a ceramic body A, and in FIG. 2, a hollow ceramic body A is housed within a holding metal fitting B. In FIG. 2, the holding metal fitting B and the hollow body A are firmly sealed by a sealing material F and a metal fitting G.
In such arrangements, however, the temperature at a portion E of the metal casing in contact with the ceramic material rises so that the metal casing B loosens because it has a larger coefficient of thermal expansion than does the ceramic material. Accordingly, the portion E of the metal casing B at which the temperature becomes high is pressed into a cylinder head C so that superheating of the cylinder head occurs. This method, however, has another disadvantage in that a large temperature gradient occurs between the ceramic material and the metal portions. Also, the tightening force due to the pressure of the ceramic structure into the cylinder head when the former is mounted in the latter as well as the thermal deformation of the head during operation has serious negative effects on the ceramic structure A.